An electric power steering apparatus is configured to detect an operation force by use of operation force detection means, and control an electric motor on the basis of the detected operation force so as to apply an assist force. In general, such an electric power steering apparatus has a fail-safe mechanism for quickly decreasing the assist force and stopping the electric motor, from the viewpoint of preventing erroneous output (application of assist force) which would otherwise occur when an anomaly occurs in the operation force detection means. However, if an anomaly occurs in the operation force detection means when the vehicle is traveling and application of assist force is stopped, the burden imposed on a driver when turning the steering wheel increases. Accordingly, it has been desired to configure the electric power steering apparatus such that, even when an anomaly occurs in the operation force detection means, the electric power steering apparatus operates the electric motor so as to continue the application of assist force to a possible extent.
Therefore, such an electric power steering apparatus that can continue application of assist force even when an anomaly occurs in the operation force detection means has been actively studied and proposed. For example, Japanese Patent Application Laid-Open (kokai) No. 2009-6985 discloses an electric power steering apparatus which continues generation of steering assist force in consideration of reaction from a road surface. This conventional electric power steering apparatus includes steering torque detection means for detecting a steering torque input to a steering system, and first torque command value computation means for computing a steering assist torque command value on the basis of the steering torque detected by the steering torque detection means. Also, this conventional electric power steering apparatus includes self-aligning estimation means for estimating a self-aligning torque transmitted from the road surface to a steering mechanism, and second torque command value computation means for computing a steering assist torque command value on the basis of the estimated self-aligning torque. The self-aligning estimation means can estimate the self-aligning torque on the basis of the steering angle of a steering mechanism, which can be calculated on the basis of the speeds of the front-side left and right wheels of the vehicle.
In this conventional electric power steering apparatus, when torque detection section anomaly detection means detects an anomaly of the steering torque detection means, the second torque command value computation means is used in place of the first torque command value computation means. Then, generation of steering assist torque is continued through use of the steering assist torque command value computed by the second torque command value computation means.
As to detection of steering angle through use of wheel speeds, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-98827 discloses a steering angle estimation apparatus for a vehicle in which rotational speeds of the four wheels of the vehicle are received from wheel rotational speed sensors provided for the four wheels; slippage of the four wheels is detected through comparison of the relation in wheel rotational speed between respective two wheels (front-side two wheels, rear-side two wheels, left-side two wheels, and right-side two wheels); and the steering angle is estimated through use of the wheel rotational speeds of the wheels other than the wheel which is slipping.
Also, for example, Japanese Patent Application Laid-Open (kokai) No. 2008-249719 discloses a steering angle estimation apparatus for a vehicle in which rotational speeds of the four wheels of the vehicle are received from wheel rotational speed sensors provided for the four wheels of the vehicle; slippage of the four wheels is detected through comparison among the rotational speed ratio between front-side left and right wheels, that between rear-side left and right wheels, that between the right-side front and rear wheels, and that between the left-side front and rear wheels; and the steering angle is estimated through use of the rotational speed ratios obtained by excluding the wheel which is slipping.
Also, for example, Japanese Patent Application Laid-Open (kokai) No. H11-78924 discloses an electronic power steering apparatus which stably supplies steering assist force to a steering system even when a steering torque sensor fails. This conventional electric power steering apparatus includes two steering torque detection means each composed of a steering torque sensor and a steering torque detector including a torque signal detector and failure detection means. In this conventional electric power steering apparatus, one steering torque detection means is switched to the other steering torque detection means in response to a failure signal from the failure detection means so as to continue the operation of the apparatus, to thereby enable the steering assist force to be supplied to the steering system on the basis of accurate steering torque.